Method for the oxidation of black liquor



April 13, 1965 A. TIRADO METHOD FOR THE OXIDATION OF BLACK LIQUOR Filed March 2, 1961 agmpressed nlllillllllllllilr #flllllIlllllltllllllllllrlillllllIII.. Il Il I l X ATTORNEY United States Patent 3,178,260 METHOD FOR THE OXIDATON F BLACK LEQUGR Adalberto Tirado, Tlaipam, Mexico City, Mexico, assigner to Faiiricas de Papel Loreto y Pena Pobre, SA., Mexico City, Mexico, a Mexican corporation Filed Mar. 2, 1961, Ser. No. 92,839 4 Claims. (Cl, 2.3-48) This invention relates to a method of and apparatus for the oxidation of sulphur compounds in solution, especially so-called black liquors, which contain suldes and rnercaptans or other sulphur-containing compounds, such as are formed in the course of paper pulp making and in petroleum reining.

The invention will be herein illustrated particularly in connection with the oxidation of sulphur compounds obtained in making paper pulp.

It is customary practice in pulp and paper making, to evaporate and dry such black liquors, and inally to calciue the dry product in order to recover, in as high a proportion as possible, the `soda and sulphur, for re-use in the pulp cooking process. As is well known, however, a substantial quantity of sulphur is lost in the form of hydrogen sulfide, mercaptans, or other organic sulphur compounds, during the evaporation or drying and calcination. Moreover, the escape of these compounds is objectionable because of their malodorous properties. Consequently, it has heretofore been the practice to oxidize the sulphur compounds in order to convert them into compounds exhibiting lower volatility, thereby decreasing both the sulphur losses and the malodors.

To that end, it has heretofore been the conventional practice to oxidize the black liquors in tanks or towers at normal atmospheric pressure, the oxidation being performed by scrubbing the liquor with air. Normally, air is added in a ratio of 100 volumes of air to 1 volume of liquor. Under these conditions, most of the sodium sulde is converted into sodium thiosulfate, the conversion of the sodium sulfide ranging from about 50 to 100%, even though conversion of sodium suliide into sodium thiosulfate theoretically requires about 1.5 liters of air per gram of sodium sulfide. Thus, liquor normally containing 5 to l0 grams sodium suliide per liter should require only about 8 to 15 liters of air per liter of liquor. It will be seen, therefore, that the quantity of air utilized in the conventional practice of oxidizing black liquor mounts to about 6 to 12 times the theoretical requirement.

Furthermore, l have found that the oxidation of mercap'rans, organic sulfides and disulfide-s is more diiiicult to obtain than the oxidation of sodium sulfide. Consequently, a substantial portion of rnercaptans and organic sulides and disuliides remains in the liquor after it has been subjected to the conventional procedure for oxidation with air at atmospheric pressure, as set forth above. it has been determined that no more than about 10 to 50% of the rnercaptans is oxidized under such conditions. Yet it is the mercaptans and organic suldes and disuldes which are the principal components that cause malodorous air pollution in the operation of kraft pulp mills.

Moreover, the conventional practice of oxidation as above set forth also requires, in order to promote contact between the air and the black liquor, the use of towers which either contain packing material or which are provided with a series of perforated trays.

The principal object of the present invention is to improve the eiciency of the oxidation of sulphur cornpounds in solution, especially black liquors containing sodium sulde, `mercaptans, and organic sullides or disuldes.

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It is a more particular object of the invention to provide a process for the oxidation of such sulphur cornpounds in solution, capable of operating so as to elfect a substantially complete oxidation ot sodium` sulfide and to effect oxidation of mercaptans to the extent ot from 50 to 100% thereof, in contrast to the l0 to 50% oxidation of mercaptans obtained in the conventional practice.

Several analytical methods are used to determine the degree or etlciency of such oxidation. The most widely used analytical methods are (l) the colorimetric method (Standard Procedure, British Columbia Forest Products, Crofton); (2) the potentiometric method (Borlew and Pascoe, T.A.P.P.., Suggested Method, Tech. Assoc. Papers 30: 570-572, .inne 1947), and (3) the conventional iodine method. Since the colorimetric method determines only sodium sulfide, while the potentiometric method determines sodium suliide plus mercaptans, and the iodine ymethod measures sodium sulfide, mercaptans, organic suldes, organic disuldes and other reducing compounds, it is a relatively simple matter to calculate the eiiiciency of the oxidation of at least the sodium sulfide and the mercaptans, when they are considered separately.

Another object of the invention is to provide a method for the oxidation of sulphur compounds in solution, as above mentioned, without requiring the use of treating towers containing the conventional packing material or series of perforated plates or trays for promoting the contact between air and the liquor.

ther objects and advantages of the invention will hereinafter appear, or will be apparent to those skilled in the art, from the following speciiications taken in conjunction with the accompanying drawing, the single figure of which illustrates diagrammatically, an arrangement of apparatus that may be utilized in the practice of the invention.

Briefly stated, the objects of the invention may be achieved by subjecting the sulphur compounds in solution, suc-h as black liquor, to oxidation by treating the same with air at super-atmospheric pressures of the order of l0 to 100 pounds per square inch while maintaining the solution at temperatures in the range ot from about 7 to, say, 220 F., the higher the temperature and pressure, the better the result being. Within this relatively broad range of temperature and pressure, operating temperatures of the order of 100 to 180 F. and -pressures of the order of 25 to 75 pounds per square inch are to be preferred in most instances.

More specifically, the oxidation is particularly advantageous when periorined at a temperature of the order of 140 F. and a pressure of about 50 pounds per square inch, since under these conditions, the volume of air utilized may be greatly reduced, it being suiiicient to use 2 or 3 times the theoretical value, instead of 6 to l2 times the same, and amounting to only 20 to 40% of the air normally used when the oxidation is performed at atmospheric pressure. At the same time, the oxidation of sodium sulfide and mercaptans remains as high as to Under these conditions, also, apparently, the oxidation of organic suldes and disulfides is also very high.

Moreover, these results may be obtained in the practice of the invention without requiring the use of the conventional series of perforated trays or plates, or of packing.

Percent Temperature Pressure Oxidation (degrees C (lb. /sq. in.) of Mereaptans Referring to the accompanying drawing, wherein is diagrammatically shown one form of a reactor and its connections for carrying out the oxidation of black liquor obtained in the operation of pulp mills, the reference numeral l designates the reactor. Hot black liquor may be pumped into the reactor through a liquor inlet pipe 2, while compressed air is introduced through an inlet pipe 3, so that a pressure of 10 to 100 pounds per square inch, and temperatures of to 220 F. may be maintained in the reactor. One or more wire mesh diusers, as indicated at 4, are preferably arranged adjacent the bottom of the reactor and connected with the air inlet pipe 3, in order to assist in mixing the air with the liquor. An automatic pressure relief valve 5, of any suitable construction, is provided on top of the reactor, in order to keep a constant pressure in the reactor and to allow the mixture of oxidized liquor and excess air to discharge through outlet 6.

In the practice of the process, a reaction time or retention time in the reactor amounting to about 2 to 4 minutes is sufficient for the air to oxidize the sodium sulfide, mercaptans and organic sultdes or disulfides.

As will be evident from the foregoing description, the invention may nd a variety of embodiments and applications in addition to those specific illustrations which have hereinabove been set forth.

What I claim is:

1. The process of treating black liquors obtained in the production of paper-making pulp and containing sodium sulphide and malodorous organic sulphides, disulphides and mercaptans, which consists essentially of oxidizing said rnalodorous constituents of the black liquor to convert them into non-malodorous compounds, said oxidation being effected by non-catalytically treating said black liquor with air at superatmospheric pressures in the range of from 10 to 100 pounds per square inch while maintaining the same at a temperature in the range of fromv 70 to 220 F., said treatment with air being effected by diffusing compressed air into said liquor While feeding the liquor and the compresesd air into one end of a reaction zone in which a substantially constant pressure within said range is maintained, and discharging from the opposite end of said zone the thus oxidized liquor containing the nonrnalodorous oxidation products of said sulphides, disulphides and mercaptans mixed within excess air.

2. The process as defined in claim 1, wherein the pressure maintained in said zone is in the range of 25 to 75 pounds per square inch super-atmospheric, and wherein said temperature is in the range of to 180 F.

3. The process as deiined in claim 1, wherein the pressure maintained in said zone is about 50 pounds per square inch super-atmospheric, and wherein said temperature is of the order of F.

4. The process as defined in claim 1, wherein the retention time of said liquor in said zone is in the range of 2 to 4 minutes.

References Cited by the Examiner UNITED STATES PATENTS 1,400,192 12/21 Whitaker 23-49 1,656,563 1/28 Koppe 23--224 1,992,611 2/ 35 Grindrod 23--224 2,464,576 3/49 Hibbard et al 208-208 X 2,472,473 6/49 Fetterly 23-224 2,644,797 7/53 Shii'ler 23--181 2,665,249 1/54 Zimmermann 210-63 2,675,297 4/ 54 Gray 23-129 2,726,927 12/55 Bergstrom et al. 23-48 2,786,873 3/57 Ohsol 260-627 X 2,805,938 9/ 57 McGauley 23-224 X FOREIGN PATENTS 353,625 7/31 Great Britain. 14,822 1884 Great Britain.

MAURICE A. BRINDISI, Primary Examiner. GEORGE D. MITCHELL, Examiner. 

1. THE PROCESS OF TREATING BLACK LIQUORS OBTAINED IN THE PRODUCTION OF PAPER-MAKING PULP AND CONTAINING SODIUM SULPHIDE AND MALODOROUS ORGANIC SULPHIDES, DISULPHIDES AND MERCAPTANS, WHICH CONSISTS ESSENTIALLY OF OXIDIZING SAID MALODOROUS CONSTITUENTS OF THE BLACK LIQUOR TO CONVERT THEM INTO NON-MALADOROUS COMPOUNDS, SAID OXIDATION BEING EFFECTED BY NON-CATALYTICALLY TREATING SAID BLACK LIQUOR WITH AIR AT SUPERATMOSPHERIC PRESSURES IN THE RANGE OF FROM 10 TO 100 POUNDS PER SQUARE INCH WHILE MAINTAINING THE SAME AT A TEMPERATURE IN THE RANGE OF FROM 70* TO 220*F., SAID TREATMENT WITH AIR BEING EFFECTED BY DIFFUSING COMPRESSED AIR INTO SAID LIQUOR WHILE FEEDING THE LIQUOR AND THE COMPRESSED AIR INTO ONE END OF A REACTION ZONE IN WHICH A SUBSTANTIALLY CONSTANT PRESSURE WITHIN SAID RANGE IS MAINTAINED, AND DISCHARGING FROM THE OPPOSITE END OF SAID ZONE THE THUS OXIDIZED LIQUOR CONTAINING THE NON-MALODOROUS OXIDATION PRODUCTS OF SAID SULPHIDES, DISULPHIDES AND MERCAPTANS MIXED WITHIN EXCESS AIR. 